Full OMNI Report (PDF)
Explanation: The graph shows EPA certified stoves to have an average efficiency of about 63% - the default number. Masonry fireplaces tested between 20% and 50% efficient whether they were tested open or with closed glass doors. The one "zero-clearance" metal fireplace tested lowest at 7% efficient - again with or without doors.
Room for Improvement: The scatter might indicate that there is a lot of room for improvement in fireplace efficiency. Fireplaces have not been tested for efficiency very often. I think clean-burning fireplaces probably will be about 63% efficiency - just as clean-burning stoves are.
Methodology: While the study compares the efficiencies of various fireplaces and stoves, it only tells half the story - maybe not even half. The method used to test for efficiency is called the "flue gas loss method". The energy lost up the chimney, which can easily be determined by the volume and temperature of the flue gasses, is deducted from the total energy known to be in the fuel burned. All the rest of the energy is assumed to be useful.
Heat Transmission: That doesn't address the vast differences in the way the heat is transmitted - or even if it's transmitted. How much heat is wasted into the brickwork if the chimney is outside the house? How much heat is wasted when cold air is sucked down the chimney to cool a light weight air cooled metal chimney? In what situations would the heat be delivered more effectively radiantly or by convection? How does the air lost up the chimney relate to the ventilation system?
The amount of heat transferred by infra-red radiation versus heat transferred by convection and/or conduction is small. The actual amount of matter heated by infra-red (ie, what you feel on your skin) is very small but because it gets delivered directly to your skin it is many times more effective. It gets heat exactly where it is needed and most easily absorbed and sensed by humans. - TiegsSummary: Fireplaces - even cheap poorly designed fireplaces - are not negatively efficient, as some would contend. In fact there is reason to believe fireplaces can be as efficient as other wood-burning appliances and can meet level-playing-field efficiency standards.
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